Method of manufacturing low alloy steel powder having a low content of oxidic constituents

ABSTRACT

MOLTEN STEEL IS ATOMIZED IN AN OXIDIZING ENVIRONMENT. THE PARTICLES THUS PRODUCED CONSIST OF A METALLIC CORE AND AN OXIDE SKIN. THE PARTICLES ARE TREATED WITH AN ACID TO REMOVE THE OXIDE SKIN, AND ARE SUBSEQUENTLY RINSED, DRIED AND ANNEALED TO PRODUCE A SUBSTANTIALLY OXIDE-FREE STEEL POWER HAVING DUCTILE PARTICLES.

Oct. 9, 1973 LlNDSKOG ET AL 3,764,295

METHOD OF MANUFACTURING LOW-ALLOY STEEL POWDER HAVING A LOW CONTENT OF OXIDIC CONSTITUENTS Filed May 5, 1972 drying milling a'romlzing fluid lreaiing with acid washing cooling drying milling oxide se p a rating annealing finished powder United States Patent 3,764,295 METHOD OF MANUFACTURING LOW-ALLOY STEEL POWDER HAVING A LOW CONTENT OF OXIDIC CONSTITUENTS Per Folke Lindskog, Anders Eric Bengtsson, and Sven- Erik Grek, Hoganas, and Lennart Yngve Lagerholm, Kungalv, Sweden, assignors t0 Hoganas AG, Hoganas, Sweden Filed May 3, 1972, Ser. No. 250,056 Claims priority, application Sweden, May 14, 1971, 6,285/71 Int. Cl. B29d 23/08 US. Cl. 75--.5 BA 7 Claims ABSTRACT OF THE DISCLOSURE Molten steel is atomized in an oxidizing environment. The particles thus produced consist of a metallic core and an oxide skin. The particles are treated with an acid to remove the oxide skin, and are subsequently rinsed, dried and annealed to produce a substantially oxide-free steel power having ductile particles.

Low-alloy steel powder is usually manufuactured by atomizing molten steel. This is generally done by spraying a liquid, usually water, or a gas, for example air or steam, under high pressure against a stream of the molten steel. The stream is thus split into drops which rapidly cool and form solid particles of steel powder. It is often advisable to collect the particles formed in this 'way in water. The powder is then separated from most of the water, for example by filtering, and is finally dried by being heated. If the powder is to be used for manufacturing machine components by the powder metalliurgical method, it must be softened by annealing in a reducing atmosphere, for example hydrogen.

During the atomization the steel is exposed to a very great extent to the surrouunding atmosphere. If, as is usual, this contains air and water vapour, the surfaces of the newly formed particles become oxidized as long as they are hot. Oxides may also be formed in the steel just before the atomization. It is thus unavoidable that these oxides accompany the steel and become atomized together with the steel. Occasionally, therefore, particles of pure oxide are formed and more often particles consisting of both oxide and steel. If, apart from iron, the steel only contains alloying elements whose oxides have lower or only negligibly higher free energy of formation than iron oxide, the oxide formation before and during the atomization is no great problem, since it is then possible to completely reduce the oxide layer to metal during annealing in reducing atmospher, for example hydrogen gas. However, many alloying elements which are extremely suit able and desirable from other points of view, form oxides having considerably higher free energy of fromation than iron oxide. Examples of such alloying elements are manganese, chromium, vanadium, titanium, boron, silicon, niobium, tantalum, beryllium and aluminium. The oxide crust formed on the particles when steel containing such alloying materials is atomized, is enriched with respect to the oxides of these alloying elements and Will not be completely reduced during the annealing. Oxides of the alloying elements (MnO, Cr O etc.) will, therefore, remain on and near the surface of the particles after the annealing. When the powder is then used in the manufacture of solid steel, for example by hot pressing porous preforms, these oxides form a substantially coherent network in the structure. This has a considerable detrimental influence on the strength of the material.

It is, therefore, extremely desirable either to prevent the ice formation of these oxides which are difilcult to reduce, or to remove them from the powder particles. The first alternative is possible, for example by atomizing the steel using a pressure medium without oxidizing properties, for example nitrogen or argon, and also protecting the steel particles from oxidation until they have cooled. Such a method is expensive, however, and is also impractical since most of the particles acquire a spherical shape which is generally not suitable for compaction.

This invention, which will be described in more detail in the following, is a solution of the problem according to the second alternative and thus makes it possible to carry out the atomizing with water or some other oxidizing agent. The invention comprises a method which is described and characterized in the claims. According to this method molten steel is atomized in the manner described in the first paragraph. The steel powder thus produced is treated with at least one inorganic or organic acid so that the oxide crust is removed from the metallic part of the particles. Most of the acid is then separated from the powder, for example by means of some form of decanting, after which the rest is removed by washing with water. The oxide residue can now be separated from the powder either while this is still wet or when it has been dried. If the oxides are nonmagnetic and the steel powder magnetic, this can be done by means of wet or dry magnetic separation. Otherwise satisfactory separation can be achieved while the powder is being washed since the oxide residue, which has relatively low density and high specific surface, has a greater tendency to be carried along by the flowing water than the heavier and more compact steel particles. In certain cases it is advisable to separate the oxide resdue by air classification after the powder has dried.

In one embodiment of the invention the powder particles are subjected to mechanical treatment before, after or both before and after the acid treatment. If the treatment is performed prior to the acid treatment, cracks are formed in the oxide crust which make it easier for the acid to penetrate to the boundary surface between the oxide layer and the metallic part of the particles and the surface layer is easily dislodged from the particles. It the powder is subjected to mechanical treatment even those oxide residues which have only been partially loosened by the acid will be removed.

Aqueous solutions of both inorganic and organic acids and mixtures thereof can be used for the treatment. Sulphuric acid, hydrochloric acid and nitric acid in concentrations of 5-10 percent of weight or mixtures of these are particularly suitable. As examples of organic acids, oxalic acid, acetic acid and formic acid may be mentioned. It is advantageous to carry out the acid treatment in a rotating drum allowing good contact between each particle and the acid.

It has been found advisable to add a pickling inhibitor to the acid. This prevents dissolution of any significant quantities of the metal.

When the powder has thus been freed from oxidic constituents, it is annealed in reducing atmosphere in order to acquire the desired ductility.

EXAMPLE 1 A steel melt consisting of 1.42% Cr, 0.94% Mn, 0.04% Si, 0.025% S, 0.014% P, 0.03% Al, 0.63% C and the remainder Fe was atomized with water in a chamber filled with nitrogen gas. During the atomizing water vapour was produced which reacted with the hot steel particles to form an oxide layer on them. The powder was collected in water, whereupon it became rapidly cooled. A small quantity (A) was taken out and dried separately. This sample proved to have an oxygen content of 0.87%. The

remainder of the powder was treated with 10% sulphuric acid for 7 minutes at 30 C., after which the acid. was decanted and the powder washed with Water. The powder was collected and dried, after which the fine and light oxide particles were separated by air classification. The acid-treated steel powder (B), the quantity of which was 83% of the quantity of molten steel, had the following composition: 1.04% Cr, 0.71% Mn, 0.02% Si, 0.022% S, 0.015% P, 0.02% Al, 0.60% C, 0.26% O.

The powders A and B were annealed at 1050 C. for 30 minutes in a gas consisting of 75% hydrogen and 25% nitrogen, after which the powder cakes, somewhat sintered, were ground down to a maximum particle size of 0.15 mm. After the annealing powder A had an oxygen content of 0.67% and powder B 0.12%.

The powder was pressed to test bodies having a density of 6.5 g./cm. which were hot forged to full density. The forged test bodies were cut up and the exposed surface ground and polished with diamond paste. The volume percentage of oxide inclusions was determined microscopically by means of linear analysis in the section surface The results can be seen from the following table:

Sample A B Density, gJcm. 7. 75 7.85 Carbon content, percent- 0. 22 0. 24 Inclusion, volume percent 0. 66 0.09

EXAMPLE 2 A steel melt consisting of 2.28% Mn, 0.03% Si, 0.021% S, 0.011% P, 0.02% A1, 0.42% C and the remainder Fe was atomized with water in a chamber filled with air. During the atomizing air and water vapour reacted with the hot particles and formed a layer of oxide on them. The powder was collected in water so that it was rapidly cooled. It was then dried and a sample (C) was taken out. It was found to have an oxygen content of 1.58%. The powder was then allowed to pass a hammer mill and afterwards treated with hydrochloric acid with 0.05% pickling inhibitor added, for 5 minutes at 25 C., after which the acid was decanted and the powder transferred to a vertical cylindrical vessel provided with an overflow at the top. Water was pumped in at the bottom and a mixture of water, acid residue and solid oxide particles flowed over the overflow. The rinsed powder was collected and dried. A sample of this powder (D), the quantity of which was 80% of the quantity of molten steel, had the following composition: 1.64% Mn, 0.02% Si, 0.018% S, 0.010% P, 0.02% Al, 0.40% C, 0.63% O. The powders C and D were annealed at 850 C. for 120 minutes in hydrogen gas, after which the slightly sintered powder cakes were ground to powder having a maximum particle size of 0.42 mm. After the annealing powder C had an oxygen content of 0.07% and powder D 0.25%. The quantity of inclusions was determined on forged test bodies as in Example 1. The results can be seen from In this case, therefore, the quantity of inclusions was reduced to one eighth of the quantity obtained when using conventional methods.

EXAMPLE 3 A steel melt consisting of 2.1% Cr, 2.0% Al, 0.82% Mn, 0.31% Mo, 0.07% Si, 0.030% S, 0.025% P, 0.55% C and the remainder Fe was atomized with water vapour. During the atomization the water vapour reacted with the hot steel particles and formed a layer of oxide on them. The powder was collected in water, where it cooled rapidly. It was then dried and a sample (B) was taken out. This proved to have an oxygen content of 1.38%. The powder was then allowed to pass a desintegrator, whereupon cracks were produced in the oxide layer on the steel particles. It was then treated with a solution of oxalic acid in water with a concentration of g./l. at 50 C. for 4 hours. The acid was decanted and the powder washed with water and dried. The powder was then allowed to pass a desintegrator again so that those oxide residues which had not loosened during the acid treatment were removed from the steel particles. The oxide particles thus dislodged were separated from the steel particles by means of magnetic separation. The powder (F) treated in this way had the following composition: 1.68% Cr, 1.05% Al, 0.59% Mn, 0.31% Mo, 0.04% Si, 0.025% S, 0.025% P, 0.54% C, 0.51% O. The powders E and F were annealed at 950 C. for 15 minutes under vacuum, after which the slightly sintered powder cakes were ground to powder having a maximum particle size of 0.175 mm. After the annealing the powder E had an oxygen content of 1.08% and the powder F 0.35%. The quantity of inclusions was determined on the forged test bodies as in Examples 1 and 2. The results are shown in the following table:

The quantity of inclusions was thus reduced to between one quarter and one fifth of the quantity of obtained with methods known hitherto. The powder is perfectly satisfactory for manufacturing sinter-forged material.

The complete process, aforesaid, is diagrammatically illustrated in the accompanying flow sheet constituting the single figure of the appended drawing.

What is claimed is:

'1. A method of manufacturing low alloy steel powder, comprising creating a stream of molten steel, creating a jet of an atomizing fluid, directing said jet in an oxidizing environment towards said stream to atomize the molten steel into particles consisting of a metallic core and an oxide skin, allowing the particles to solidify and cool, treating the powder thus produced with an aqueous solution of an acid to remove the oxide skin from the metallic core, rinsing, collecting and drying the metallic powder thus produced, and annealing the dry metallic powder to produce a substantially oxide-free steel powder having ductile particles.

2. A method as claimed in claim 1, comprising exposing the powder, before the treatment with the acid, to a mechanical treatment to produce cracks in the oxide skin of the particles thus facilitating the penetration of the acid through the oxide skin.

3. A method as claimed in claim 1, comprising exposing the powder, after the treatment with the acid, to a mechanical treatment to detach any remaining oxide skin residues adhering to the metallic core.

4. A method as claimed in claim 1, comprising treating the powder with an aqueous solution containing an acid and a pickling inhibitor, to reduce the attack of the c d on he m ta l c co e 3,764,295 5 6 5. A method as claimed in claim 1, in which the molten References Cited steel is a low alloyed steel containing, in addition to iron, UNITED STATES PATENTS at least one alloying element selected from the group com- 2,784,073 3/1957 Miehalke 750.5 BA prlsmg Mn, Cr, V, T1, B, S1, Nb, Ta, Be and Al, in a total 2,861,879 11/1958 Michalke 75 0'5 BA quantity of up to 10 percent by welght. 5 3 078 158 2/1963 Naeser 5 BA 6. Method according to claim 1, characterized in that t 3,676,103 7/1972 LeBrasse et al. 750.5 BA the powder particles are separated from the oxides by magnetic separation. WAYLAND W. STALLARD, Primary Examiner 7. Method accordlng to claim 1, characterized in that the powder particles are separated from the oxides with 10 US. Cl. X.R. the of an air classifier. 

